The present invention relates to the field of light-emitting diodes, and more particularly the field of monolithic light-emitting diodes.
Such diodes are, for example, known from the application U.S. Pat. No. 6,445,009, wherein a diode is disclosed, which comprises a device including a substrate whereon a matrix is positioned, which comprises at least a stack of quantum wells of the GaN or GaInN type emitting in the visible spectrum at an ambient temperature in a layer of AlN or GaN respectively. In the application U.S. Pat. No. 6,445,009, a system of (Al, Ga, In)N or III-V nitrides materials is used, and these are semiconductors having a wide bandgap, and which have the characteristic of being able to emit in the whole visible spectrum. For example, a white light is obtained by mixing in the active zone of the diode, i.e. in the zone through which the current of the diode passes, quantum wells or InGaN/(Al)GaN quantum dots emitting in the blue and other ones emitting in the yellow.
Besides, it is also admitted that III-V nitrides are efficient materials for the production of monolithic light-emitting diodes. However, it is known that the output of emissions through electric pumping varies according to the emitted wavelength and more particularly, that the output in the blue is twice as good as the output in the yellow. This is the reason why the diodes emitting in the blue are the most widely used on the market today. The light output in the diode emitting in the blue and in the yellow is thus limited by the properties of the quantum dots or the quantum wells emitting in the yellow.
A first aim of the present invention is not to be limited by light outputs through an electric injection at different wavelengths. In addition, it is known that in the electric injection diodes using quantum dots or quantum wells, the distribution of the electrons and the holes in the quantum dots or in the quantum wells is modified as a function of the voltage applied to the diode. The colour emitted may thus vary with the intensity of the electric current. This characteristic may be useful if diodes which can change colours are desired, but it is a drawback for the emission in the white when the purpose is lighting.
A second aim of the present invention is thus to avoid the variation of the colour emission as a function of the intensity of the current of the diode. In the field of non monolithic diodes, white diodes are widely used, which are composed of a first monolithic part emitting in the blue (blue diode), above which a phosphorescent material is positioned which absorbs a part of the blue photons emitted by the blue diode and reemits yellow photons, the combination of such two lights giving a white light.
However, such two-part diodes of the standard blue diode-phosphor type have, on the one hand, the drawback that the phosphor positioned above the blue diode results in a deterioration of the general performances of the device over time. This leads to a deterioration of the white light over time and to a limitation of the life of the white LED with respect to a standard blue LED. On the other hand, it should be noted that the manufacturing of such a white LED requires a first conventional step of epitaxial growth of the blue LED and an additional step of the deposition of phosphor. Such additional step of the deposition thus involves higher production costs.
Another aim of the present invention is thus to provide a white diode through epitaxial growth only, i.e. a monolithic diode. Another aim of the present invention is thus to provide a white diode, the emission properties of which are stable over time.
The application US 2003/006430 which discloses a diode comprising Si- or Se-doped GaN layers making it possible to obtain an emission in the yellow. It is also known that in such layers, the emission is caused by deep levels resulting from crystalline defects. The quantum efficiency of this type of layer is thus limited. Besides, such deep levels emit at a wavelength which is fixed in the yellow. The diode of the above-mentioned document does not make it possible to obtain a diode which can emit in the whole visible spectrum by combining the colours between the light emitted in the active zone and the light emitted in the passive zone.
The application 2002/0139984 is also known, which describes a diode comprising a stack of semi-conductive layers in the passive zone of the diode. Such a stack of layers is made so as to compose a mirror for the wavelength emitted by the active zone, in order to increase the photons extraction efficiency. Besides, such layers must have an emission wavelength very close to that of the active zone for example less than 0.9 times half the width of the spectrum. Thus, for example for a blue LED at 450 nm and a width of 20 nm, the emission wavelength of such layers is comprised between 450 and 459 nm. The result is that the function of the semi-conductive layers in the above-mentioned document is not to denature the colour of the active zone, so that for example a blue LED remains blue, while increasing the extraction efficiency by using the mirror properties of the stack of quantum wells. Such a diode thus does not make it possible to make colour combinations between the light emitted by the active zone and the light emitted by the passive zone.
Another aim of the invention is to provide a diode which can potentially emit in the whole visible spectrum and more particularly when it is excited by a blue LED. At least one of these aims is reached according to the present invention by a device comprising a matrix made of III-V nitride, said matrix comprising at least an active first portion through which an electric current passes and at least a passive second portion through which no electric current passes, said matrix comprising at least a first zone comprising a first stack of quantum wells or planes of quantum dots of III-V nitride, said first zone being positioned in said active first portion and at least a second zone comprising a second stack of quantum wells or planes of quantum dots of III-V nitride, characterised in that said second zone is positioned in said passive portion of said matrix. Thanks to the stacks of quantum wells or quantum dots respectively positioned in the active portion and the passive portion of the device, it is possible to control efficiently the lights emitted by the first zone and the second zone in order to generate, at the device output, a light which can spread on the whole visible spectrum.
According to the invention, the first zone comprising a first stack of quantum wells or planes of quantum dots of III-V nitride forms a first quantum confinement and a second zone comprising a second stack of quantum wells or planes of quantum dots of III-V nitride forms a second quantum confinement. By positioning the second quantum confinement zone in the passive portion of the monolithic matrix, the present invention solves the problems relating to the difference in the emission as a function of the currents which go through two different quantum zones in the known diodes. Thus, in the device according to the present invention, the second quantum confinement zone positioned in the passive zone will in fact be optically pumped by the photons emitted by the first quantum confinement zone, the latter being electrically pumped by the current of the diode passing in the active zone.
The optical pumping of the second zone in the passive portion of the matrix of the device thus makes it possible to avoid the drawbacks connected to the electric pumping, and more particularly the dependence of the emission on the current intensity. Besides, the monolithic constitution of the matrix comprising the passive zone and the active zone in one III-V nitride material, makes it possible to obtain the device according to the invention with only one step of epitaxial growth. The distribution of the III-V nitride elements in the matrix is carried out according to the invention, so that said first zone forms a quantum confinement and that the second zone forms a quantum confinement, i.e. the part of the matrix between such zones forms a quantum gate between such zones. This is obtained, in a manner known per se, by selecting the III-V nitride materials as a function of the bandgaps of such materials.
In order to be able to adapt, in particular, the colour of the light emitted by the first zone, said at least one first zone is able to emit photons at least a first wavelength through an electric injection by said current passing in said active zone, said at least first wavelength being determined by the dimensions of said first stack of quantum wells or planes of quantum dots of III-V nitride and the composition of said first stack of quantum wells or planes of quantum dots of III-V nitride. Similarly, said at least second zone is able to emit photons at least a second wavelength through an optical pumping by said photons emitted by said first zone, said at least one second wavelength being determined by the dimensions of said second stack of quantum wells or planes of quantum dots of III-V nitride and the composition of said second stack of quantum wells or planes of quantum dots of III-V nitride.
In order to diffuse a white light and more particularly for lighting applications, said first zone and said second zone are selected so that the combination of the light signal corresponding to the photons at said wavelength and the light signal corresponding to the photons of said second wavelength diffuses a substantially white light. In order to reach a good output of light emissions, said at least one first zone is able to emit photons in the blue through an electric injection by said current passing in said active zone. In this case, in order to obtain a white light at the output of the device, said at least one second zone is able to emit photons in the yellow by an optical pumping by said photons emitted by said first zone. The emission in the yellow makes it possible to use only one stack of quantum wells or planes of quantum dots.
According to another embodiment, which makes it possible to improve the colour temperature and the colour rendering index while obtaining a white light at the device output, said at least one second zone is able to emit photons in the green and in the red by an optical pumping by said photons emitted by said first zone. According to an alternative solution of the invention, said first zone is composed of a stack of quantum wells of the InGaN/GaN type. In such embodiment of the invention, said second zone is composed of a stack of planes of quantum dots of the GaN/AlN type. In order to allow the passage of the current towards the first active zone while making the quantum confinement of the first active zone, said matrix made of III-V nitride includes a third conductive portion which forms a quantum gate for said first zone.
The invention also relates to a light-emitting diode comprising a device such as previously described and means for generating said current, said means for generating said current being so arranged as to define said active first portion through which said electric current passes and said passive second portion through which no electric current passes.